Linked List Cycle II

 1     ListNode *detectCycle(ListNode *head) {
 2         if(!head||!head->next)
 3             return NULL;
 4         ListNode *fast,*slow;
 5         fast=head;
 6         slow=head;
 7         while(fast&&fast->next)
 8         {
 9             slow=slow->next;
10             fast=fast->next->next;
11             if(fast==slow)
12             {
13                 fast=head;
14                 while(fast!=slow)
15                 {
16                     fast=fast->next;
17                     slow=slow->next;
18                 }
19                 return fast;
20             }
21         }
22         return NULL;
23     }